Porous Coordination Polymer with Flexibility Imparted by Coordinatively Changeable Lithium Ions on the Pore Surface

UoC-7: A Bimetallic K-Zn-MOF with an Anionic Framework Based on Fluorinated Trimesate Ligands Exhibiting a Large CO2 Uptake

In solvothermal reactions of Zn(NO3)2×6H2O with K(H2mF-BTC) or K(H2dF-BTC) in DMF/ethanol or DMA/ethanol solvent mixtures, single crystals of the MOFs UoC-7(1F) and UoC-7(2F) were obtained crystallizing in the hexagonal space group P63/m (no. 176) (H3BTC: 1,3,5-benzenetricarboxylic acid; mF-/dF: mono-/difluoro; DMF: N,N-dimethylformamide; DMA: N,N-dimethylacetamide; UoC: University of Cologne). According to the general composition [(CH3)2NH2][K2Zn3(mF-/dF-BTC)3(H2O)]×solvent, UoC-7 consists of an anionic bimetallic framework. The charge is compensated by a (CH3)2NH2 + cation stemming from the (partial) hydrolysis of the solvent. The crystal structure shows large channels along the hexagonal [001] direction, which accommodate the cations as well as solvent molecules. Surface areas (SBET) of 2740 m2/g (UoC-7(1F)) and 1643 m2/g (UoC-7(2F)) were obtained from N2 sorption measurements. UoC-7 shows structural similarities to the MOF NKU-521 with a 5-(1H-tetrazol-5-yl)isophthalate linker. Both MOFs exhibit a 4,7,8T14 topology. Despite smaller channels in UoC-7 compared to NKU-521, the CO2 uptake is considerably higher (~164 cm3/g at 1 bar/293 K) being one of the highest CO2 uptakes observed up to now.

Significance of an Environmental Gas Cell to Obtain a Fully Dehydrated Form and CO2-Pressurized Structure of a Metal-Organic Framework Using In Situ Single-Crystal X-ray Diffraction at 298 K

The single-crystal X-ray diffraction method was employed to characterize a rigid hydrated metal-organic framework (MOF), [Co₂(MA)(INA)·2H₂O]_n, that displays an affinity toward water molecules under ambient conditions after dehydration. The fully dehydrated form was obtained using an environmental gas cell technique in a stepwise manner followed by its CO₂-pressurized structure at 298 K using in situ crystallography.

Bimetallic Rod-Packing Metal-Organic Framework Combining Two Charged Forms of 2-Hydroxyterephthalic Acid

Although many rod-packing metal-organic frameworks are known, few are based on ordered heterometallic rod building unit. We show here the synthesis of CPM-76 based on an unprecedented Zn-Mg bimetallic rod with crystallographically distinguishable metal sites. The configuration of the rod offers two types of coordination site with trigonal bipyramidal and octahedral sites selectively occupied by Zn and Mg, respectively. Also unusual is the inter-connection mode between the rods, which is based on dual-charged forms (-3 and -2) of the 2-hydroxyterephthalic acid (H₃ OBDC) ligand. Interestingly, each metal site in CPM-76 binds one solvent molecule, leading to a high density of solvent binding sites.

Improved Predictive Tools for Structural Properties of Metal-Organic Frameworks

The accurate determination of structural parameters is necessary to understand the electronic and magnetic properties of metal-organic frameworks (MOFs) and is a first step toward accurate calculations of electronic structure and function for separations and catalysis. Theoretical structural determination of metal-organic frameworks is particularly challenging because they involve ionic, covalent, and noncovalent interactions, which must be treated in a balanced fashion. Here, we apply a diverse group of local exchange-correlation functionals (PBE, PBEsol, PBE-D2, PBE-D3, vdW-DF2, SOGGA, MN15-L, revM06-L, SCAN, and revTPSS) to a broad test set of MOFs to seek the most accurate functionals to study various structural aspects of porous solids, in particular to study lattice constants, unit cell volume, two types of pore size characteristics, bond lengths, bond angles, and torsional angles). The recently developed meta functionals revM06-L and SCAN, without adding any molecular mechanics terms, are able to predict more accurate structures than previously recommended functionals, both those without molecular mechanics terms (PBE, PBEsol, vdW-DF2, and revTPSS) and those with them (PBE-D2 and PBE-D3). To provide a broader test, these two functionals are also tested for lattice constants and band gaps of unary, binary, and ternary semiconductors.

Spontaneous enanti-omorphism in poly-phased alkaline salts of tris-(oxalato)ferrate(III): crystal structure of cubic NaRb5[Fe(C2O4)3]2

We show here that the phenomenon of spontaneous resolution of enanti-omers occurs during the crystallization of the sodium and rubidium double salts of the transition metal complex tris-(oxalato)ferrate(III), namely sodium penta-rubidium bis-[tris-(oxalato)ferrate(III)], NaRb5[Fe(C2O4)3]2. One enanti-omer of the salt crystallizes in the cubic space group P4332 with Z = 4 and a Flack absolute structure parameter x = -0.01 (1) and its chiral counterpart in the space group P4132 with x = -0.00 (1). All metal ions are at crystallographic special positions: the iron(III) ion is on a threefold axis, coordinated by three oxalate dianions in a propeller-like conformation. One of the two independent rubidium ions is on a twofold axis in an eightfold coordination with neighbouring oxalate oxygen atoms, and the other one on a threefold axis in a sixfold RbO6 coordination. The sodium ion is at a site of D3 point group symmetry in a trigonal-anti-prismatic NaO6 coordination.

Computational Screening of MOFs for Acetylene Separation

Efficient separation of acetylene (C₂H₂) from CO₂ and CH₄ is important to meet the requirement of high-purity acetylene in various industrial applications. Metal organic frameworks (MOFs) are great candidates for adsorption-based C₂H₂/CO₂ and C₂H₂/CH₄ separations due to their unique properties such as wide range of pore sizes and tunable chemistries. Experimental studies on the limited number of MOFs revealed that MOFs offer remarkable C₂H₂/CO₂ and C₂H₂/CH₄ selectivities based on single-component adsorption data. We performed the first large-scale molecular simulation study to investigate separation performances of 174 different MOF structures for C₂H₂/CO₂ and C₂H₂/CH₄ mixtures. Using the results of molecular simulations, several adsorbent performance evaluation metrics, such as selectivity, working capacity, adsorbent performance score, sorbent selection parameter, and regenerability were computed for each MOF. Based on these metrics, the best adsorbent candidates were identified for both separations. Results showed that the top three most promising MOF adsorbents exhibit C₂H₂/CO₂ selectivities of 49, 47, 24 and C₂H₂/CH₄ selectivities of 824, 684, 638 at 1 bar, 298 K and these are the highest C₂H₂ selectivities reported to date in the literature. Structure-performance analysis revealed that the best MOF adsorbents have pore sizes between 4 and 11 Å, surface areas in the range of 600–1,200 m²/g and porosities between 0.4 and 0.6 for selective separation of C₂H₂ from CO₂ and CH₄. These results will guide the future studies for the design of new MOFs with high C₂H₂ separation potentials.

Mixed-Lanthanide Porous Coordination Polymers Showing Range-Tunable Ratiometric Luminescence for O2 Sensing

Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for self-referenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eu_x), were synthesized by solvothermal reactions. Single-crystal X-ray diffraction revealed that MCF-53(Tb/Eu_x) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF-53(Tb/Eu_x) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.

Evolution from linear tetranuclear clusters into one-dimensional chains of Dy(iii) single-molecule magnets with an enhanced energy barrier

Structural evolution from Dy₄ clusters into 1D Dy(iii)-chains, the anisotropy energy barrier is enhanced from 45.4(2) to 144.2(1) K.

Theoretical study of the gas-phase decomposition of Pb[(C6H5)2PSSe]2 single-source precursor for the chemical vapour deposition of binary and ternary lead chalcogenides

A theoretical study of Pb(II) square planar thioselenophosphinate, Pb[(C6H5)2PSSe]2 precursor for the chemical vapour deposition process of preparing lead chalcogenides, has been carried out. The geometries of the species were optimized by employing the density functional theory. The transition states as well as the linked intermediates linking them were confirmed with frequency analyses. The density functional theory calculation (M06/LACVP*, gas) reveals that kinetically, the steps that lead to PbSe formation are more favourable than those that lead to PbS and ternary PbSexS1−x formation on both the singlet and the doublet potential energy surface. However, thermodynamically, the steps that lead to ternary PbSexS1−x formation are more favourable than those that lead to PbSe and PbS formation on the doublet potential energy surface. On the singlet potential energy surface, an alternative route leading to the formation of ternary PbSexS1−x is more favoured on both thermodynamic and kinetic grounds than those that lead to PbSe and PbS formation. The computational studies indicate that the decomposition of the complex in chemical vapour deposition may involve more than one step, and thus the formation of ternary PbSexS1−x is a result of thermodynamic and kinetic factors in controlling the material formed during the deposition process.

Structurally simple complexes of CO2

A wide range of structurally characterized adducts of CO₂are discussed in this review, from the strongly bound, charge assisted carbamate complexes through the weaker halide and pseudo-halide complexes to the weakest possible inclusion complexes.

The three-dimensional coordination polymer poly[[aqua[μ4-2,2'-(diazene-1,2-diyl)dibenzoato]lead(II)] 1,2-bis(pyridin-4-yl)ethylene hemisolvate]

A novel three-dimensional coordination polymer, {[Pb(C14H8N2O4)(H2O)]·0.5C12H10N2}n, has been synthesized by hydrothermal reaction of Pb(OAc)2·3H2O (OAc is acetate), 2,2'-(diazene-1,2-diyl)dibenzoic acid (H2L) and 1,2-bis(pyridin-4-yl)ethylene (bpe). The asymmetric unit contains a crystallographically independent Pb(II) cation, one L(2-) ligand, an aqua ligand and half a bpe molecule. Each Pb(II) centre is seven-coordinated by six O atoms of bridging-chelating carboxylate groups from L(2-) ligands and by one O atom from a coordinated water molecule. The Pb(II) cations are bridged by L(2-) ligands, forming [PbO2]n chains along the a axis. These chains are further connected by L(2-) ligands along the b and c axes to give a three-dimensional framework with a 4(12)6(3) topology. The channel voids are occupied by bpe molecules.

Crystallographic studies of gas sorption in metal-organic frameworks

Metal-organic frameworks (MOFs) are a class of porous crystalline materials of modular design. One of the primary applications of these materials is in the adsorption and separation of gases, with potential benefits to the energy, transport and medical sectors. In situ crystallography of MOFs under gas atmospheres has enabled the behaviour of the frameworks under gas loading to be investigated and has established the precise location of adsorbed gas molecules in a significant number of MOFs. This article reviews progress in such crystallographic studies, which has taken place over the past decade, but has its origins in earlier studies of zeolites, clathrates etc. The review considers studies by single-crystal or powder diffraction using either X-rays or neutrons. Features of MOFs that strongly affect gas sorption behaviour are discussed in the context of in situ crystallographic studies, specifically framework flexibility, and the presence of (organic) functional groups and unsaturated (open) metal sites within pores that can form specific interactions with gas molecules.

Towards imaging electron density inside metal-organic framework structures

Herein, we present electron density maps of three MOFs with different guests or post-synthetic modifications produced using single crystal X-ray data from laboratory diffractometers. Analysis of the electron density maps reveals possible differences inside the pores indicating that this approach may be used to explore frameworks using inexpensively gained X-ray data.

Single-crystal X-ray diffraction studies on structural transformations of porous coordination polymers

This review gives a brief overview of single-crystal X-ray diffraction studies and single-crystal to single-crystal transformations of porous coordination polymers.

Rhombus-shaped tetranuclear [Ln4] complexes [Ln = Dy(III) and Ho(III)]: synthesis, structure, and SMM behavior

The reaction of a new hexadentate Schiff base hydrazide ligand (LH3) with rare earth(III) chloride salts in the presence of triethylamine as the base afforded two planar tetranuclear neutral complexes: [{(LH)2Dy4}(μ2-O)4](H2O)8·2CH3OH·8H2O (1) and [{(LH)2Ho4}(μ2-O)4](H2O)8·6CH3OH·4H2O (2). These neutral complexes possess a structure in which all of the lanthanide ions and the donor atoms of the ligand remain in a perfect plane. Each doubly deprotonated ligand holds two Ln(III) ions in its two distinct chelating coordination pockets to form [LH(Ln)2](4+) units. Two such units are connected by four [μ2-O](2-) ligands to form a planar tetranuclear assembly with an Ln(III)4 core that possesses a rhombus-shaped structure. Detailed static and dynamic magnetic analysis of 1 and 2 revealed single-molecule magnet (SMM) behavior for complex 1. A peculiar feature of the χM" versus temperature curve is that two peaks that are frequency-dependent are revealed, indicating the occurrence of two relaxation processes that lead to two energy barriers (16.8 and 54.2 K) and time constants (τ0 = 1.4 × 10(-6) s, τ0 = 7.2 × 10(-7) s). This was related to the presence of two distinct geometrical sites for Dy(III) in complex 1.